The present invention is directed to an electrical charging device that can recharge either or both of secondary batteries or capacitors, and which implements various intelligent charge-discharge methods for automatically preventing the onset of memory effects in NiCd batteries. A power factor correction circuit for use in a power pack is also disclosed.
New electro-chemical capacitors (xe2x80x9csuper-capacitorsxe2x80x9d or double-layer capacitors) are currently available in sizes that can compete in some applications with small Nickel-Cadmium (NiCd) and Nickel-Metal-Hydride (NiMH) batteries. Secondary (rechargeable) batteries store energy in the dissociation and recombination of particular chemical compounds. Super-capacitors store energy in the electric field of ionic compounds in close proximity. Secondary batteries offer higher energy density than super-capacitors. For example, there exist conventional 2.3V, 100 Farad super-capacitors that are approximately the size of two xe2x80x9cAAxe2x80x9d batteries. The energy stored in such a super-capacitor is provided by the well-known formula: pgCV2/2=100F (2.3V)2/2=264 Joules. Standard xe2x80x9cAAxe2x80x9d NiCd batteries have a rating of 1.2V at 0.6 Ampere-hour, so two xe2x80x9cAAxe2x80x9d batteries have a total capacity of 2(1.2V)(0.6A-hr)(3600 sec/hr)=5184 Joules.
However, while conventional batteries have a significantly higher energy storage density, super-capacitors have superior peak current capability. For example, the same super-capacitor discussed previously has a peak current rating of over 20 Amperes while the two-xe2x80x9cAAxe2x80x9d NiCd pack has a current rating of only 2.4 A. Super-capacitors also have a superior charge-discharge rated life. A typical super-capacitor can be charged and discharged over 100,000 times while the lifetime of a typical NiCd battery is only 500-1000 charge-discharge cycles.
Super-capacitors also do not suffer from the xe2x80x9cmemory effectxe2x80x9d that occurs in NiCd batteries. The memory effect is a well-known characteristic of NiCd batteries in which repeated partial discharges of the battery cause part of the energy in the battery to become inaccessible during discharge. This is a reversible change in the battery and can be corrected by periodically fully-discharging or xe2x80x9cdeep dischargingxe2x80x9d the battery. Deep discharges are generally performed by removing the battery from the battery-powered device, discharging the battery, and then recharging it on a dedicated external battery charger. However, this solution is not always practical. Alternatively, deep discharge can be performed by running the powered equipment until the battery is fully discharged. However, this solution is also not suitable in many situations because the battery may die unexpectedly, and thus operation of the powered device will be unreliable. Although battery xe2x80x9clifetimexe2x80x9d indicators may provide some warning, they often operate unreliably when there is an existing memory effect which must be countered. In addition, some electronic devices may be damaged as the battery voltage drops towards the end of the deep-discharge cycle.
Another difference between batteries and super-capacitors is the operating voltage range. Batteries have a narrow operating voltage range. For example, 90% of the energy in a NiCd battery is supplied over a voltage range of 1.35 to 1.05V. In a conventional 2.3V super-capacitor, this range is from 2.3V to 0.7V. Thus, batteries are much easier to use in powering systems because the voltage is generally constant and the battery output is about the same at the end of the discharge cycle as at the beginning.
Because of the various technical differences between batteries and super-capacitors, each device must be charged in specific, often incompatible, ways. Thus, hardware designers must select which rechargeable device to use in powering equipment, making tradeoffs between the longer charge/discharge life and low-maintenance requirements of super-capacitors and the higher stored energy density and more constant output voltage of secondary batteries. The selection is important, particularly if the equipment includes a built-in charger.
It is an object of the invention to provide a power charger which charges and provides conditioned power from either or both of a super-capacitor and a secondary battery. The choice of energy device would preferably be at the user""s discretion, so that one user could select super-capacitors because of their much longer charge/discharge life and low-maintenance requirements, while another user could choose secondary batteries because of their higher stored energy, while a third user may select a combination of the two.
It is another object of the invention to provide an embedded charging system in which the charger is included as part of a larger system performing some function with the stored energy, and where the charger includes an on-board automatic system for performing periodic deep-discharge maintenance of NiCd batteries to remove the memory effect.
Yet another object of the invention is to provide a charger which automatically detects the storage element type and automatically or manually adjusts the recharging parameters to accommodate the particular storage element.
It is a further object of this invention to provide a xe2x80x9cpre-conditioningxe2x80x9d element that narrows the apparent charge and discharge voltage range of super-capacitor elements.